The current semiconductor manufacturing process for Very Large Scale Integrated circuits ("VLSI circuits") uses a single wafer of semiconductor material. In a VLSI circuit, many duplicate devices are simultaneously fabricated on the surface of the wafer. The fabrication process typically involves as many as sixty stages of chemically processing the wafer's surface. Between each stage of the processing, the chemical used in the previous stage must be thoroughly washed or cleansed from the wafer surface in a washing step. Ultrapure water is used in the washing step.
The volume of ultrapure water required to wash the chemicals from a single wafer for all stages of processing may total as much as 1000 liters. Any nonvolatile residue such as fine particles, microorganisms, and dissolved impurities have the potential to remain on the wafer surface after the water has evaporated. Since minute traces of residue material on a wafer surface can cause defects in the resulting semiconductor device, it is imperative to use ultrapure water of the highest quality/purity to limit possible defects. Therefore, instrumentation is required to monitor ultrapure water quality at nonvolatile residue quantities of a few parts per billion (hereafter referred to for convenience as "PPB").
An instrument capable of measurements of this nature is described in U.S. Pat. No. 4,761,074 issued to Kohsaka et al. Kohsaka et al discloses a method and apparatus for measuring impurity concentrations in a liquid. The apparatus includes an atomizer for atomizing the liquid by mixing it with clean air and generating droplets of a predetermined size distribution. An evaporator evaporates the fine droplets, thereby generating nonvolatile residue particles. A condensation nucleus counter (hereinafter referred to for convenience as "CNC") then counts the number of fine nonvolatile residue particles. A single processing unit measures the nonvolatile residue concentration of the liquid based on the sensitivity characteristic of the CNC, the distribution of the droplet size generated by the atomizer, and the number of particles counted by the CNC.
Another method and apparatus for measurement of impurities in liquids is disclosed in U.S. Pat. No. 4,794,086 issued to Kasper et al. The method includes dispersing the test liquid (i.e., the liquid to be measured for nonvolatile residue) into uniform droplets of a precisely known diameter in a gas stream using a vibrating orifice generator to disperse the liquid. The droplets are then evaporated leaving a nonvolatile residue particle having a known diameter. The static charge on the droplets and/or residue particles is then neutralized and the diameter of the residue particles is then measured. The residue concentration by volume within the liquid can then be calculated. The residue particles are sized by an optical particle counter, or alternatively a differential mobility size analyzer, in order to determine the impurity level.
Both of the systems described above involve dispersing the liquid to be measured for nonvolatile residue into droplets by using an atomization process. The droplets are then evaporated to leave a residue particle which is counted or sized. However, these systems do not allow for continuous control of the feed rate of the ultrapure liquids at an optimum flowrate as provided in the present invention.
Traditionally, a peristaltic pump with collapsible tubing has been used to feed the ultrapure water to a nozzle for atomization. A cylindrical ball bearing is used in conjunction with the pump and collapsible tubing to rotate and collapse the tubing to obtain the proper flowrate. Use of this type of pump system does not allow utilization of the industry standard materials for use with ultrapure water. Therefore, using this type of system introduces impurities into the ultrapure water through the pump system. Use of a syringe pump introduces the same type of contaminants as the use of the pump and collapsible tubing described above. Additionally, the above described systems do not provide a means for altering the operating range of the nonvolatile residue monitor.
The present invention addresses the above described problems associated with the apparatus and method of measuring nonvolatile residue in liquids. The apparatus operates in a more accurate, more controlled manner with an increased response time. The system preferably utilizes noncontaminating materials in its construction and includes means for controlling the liquid flow at a very low flowrate thereby eliminating the introduction of impurities into the ultrapure water. The system then utilizes means for accurately measuring the nonvolatile residue in the liquid. The system also uses a variable number of diffusion screens to alter the operating range of the nonvolatile residue monitor and allows collection of the residue for identification.